問題描述

我正在嘗試制作一個增長率計算器(Double)，它將結果四舍五入到最接近的整數并從那里重新計算，如下所示:

I'm trying to make a calculator of growth rate (Double) that will round the result to the nearest Integer and recalculate from there, as such:

let firstUsers = 10.0
let growth = 0.1
var users = firstUsers
var week = 0


while users < 14 {
    println("week (week) has (users) users")
    users += users * growth
    week += 1
}

但到目前為止我一直做不到.

but I've been unable so far.

編輯我是這樣做的:

var firstUsers = 10.0
let growth = 0.1
var users:Int = Int(firstUsers)
var week = 0


while users <= 14 {
    println("week (week) has (users) users")
    firstUsers += firstUsers * growth
    users = Int(firstUsers)
    week += 1
}

雖然我不介意它總是四舍五入，但我不喜歡它，因為 firstUsers 必須成為變量并在整個程序中更改(以便進行下一次計算)，我不希望它發生.

Although I don't mind that it is always rounding down, I don't like it because firstUsers had to become a variable and change throughout the program (in order to make the next calculation), which I don't want it to happen.

推薦答案

Foundation 庫中有一個round 可用(其實在Darwin，但是 Foundation 導入 Darwin 并且大多數時候你會想要使用 Foundation 而不是使用 Darwin 直接).

There is a round available in the Foundation library (it's actually in Darwin, but Foundation imports Darwin and most of the time you'll want to use Foundation instead of using Darwin directly).

import Foundation

users = round(users)

在操場上運行您的代碼，然后調用:

Running your code in a playground and then calling:

print(round(users))

輸出:

15.0

round() 總是在小數位為 >= .5 時向上舍入，在 時總是向下舍入..5(標準舍入).您可以使用 floor() 強制向下舍入，使用 ceil() 強制向上舍入.

round() always rounds up when the decimal place is >= .5 and down when it's < .5 (standard rounding). You can use floor() to force rounding down, and ceil() to force rounding up.

如果需要四舍五入到特定的地方，那么你乘以pow(10.0, number of places)，round，再除以pow(10，名額):

If you need to round to a specific place, then you multiply by pow(10.0, number of places), round, and then divide by pow(10, number of places):

四舍五入到小數點后兩位:

Round to 2 decimal places:

let numberOfPlaces = 2.0
let multiplier = pow(10.0, numberOfPlaces)
let num = 10.12345
let rounded = round(num * multiplier) / multiplier
print(rounded)

輸出:

10.12

注意:由于浮點數學的工作方式，rounded 可能并不總是完全準確.最好將其更多地考慮為舍入的近似值.如果您這樣做是為了顯示，最好使用字符串格式化來格式化數字，而不是使用數學來舍入它.

Note: Due to the way floating point math works, rounded may not always be perfectly accurate. It's best to think of it more of an approximation of rounding. If you're doing this for display purposes, it's better to use string formatting to format the number rather than using math to round it.

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